In a data communication system it is necessary to isolate the terminal apparatus from the transmission line so as to protect the terminal apparatus and those who use it from line hazards such as lightning. Typically, electromechanical relays or opto-electric isolation circuits are used for this function; and it is known in the art to provide polar or neutral interfacing with such relays or opto-electric isolators. For example, a Form C style electromagnetic relay may be used to provide a three state keyer. Relays, however, tend to be slow, cumbersome, expensive and a considerable current drain. While many of the problems of relays are avoided or reduced by the use of opto-electric isolators, such devices still require fairly complicated circuitry for their operation and cause a significant current drain in their operation.